Function unfilterScanline

dependency/densecrf/examples/lodepng.cpp:3963–4035

}

static unsigned unfilterScanline(unsigned char* recon, const unsigned char* scanline, const unsigned char* precon,
                                 size_t bytewidth, unsigned char filterType, size_t length)
{
    /*
     For PNG filter method 0
     unfilter a PNG image scanline by scanline. when the pixels are smaller than 1 byte,
     the filter works byte per byte (bytewidth = 1)
     precon is the previous unfiltered scanline, recon the result, scanline the current one
     the incoming scanlines do NOT include the filtertype byte, that one is given in the parameter filterType instead
     recon and scanline MAY be the same memory address! precon must be disjoint.
     */
    
    size_t i;
    switch(filterType)
    {
        case 0:
            for(i = 0; i != length; ++i) recon[i] = scanline[i];
            break;
        case 1:
            for(i = 0; i != bytewidth; ++i) recon[i] = scanline[i];
            for(i = bytewidth; i < length; ++i) recon[i] = scanline[i] + recon[i - bytewidth];
            break;
        case 2:
            if(precon)
            {
                for(i = 0; i != length; ++i) recon[i] = scanline[i] + precon[i];
            }
            else
            {
                for(i = 0; i != length; ++i) recon[i] = scanline[i];
            }
            break;
        case 3:
            if(precon)
            {
                for(i = 0; i != bytewidth; ++i) recon[i] = scanline[i] + (precon[i] >> 1);
                for(i = bytewidth; i < length; ++i) recon[i] = scanline[i] + ((recon[i - bytewidth] + precon[i]) >> 1);
            }
            else
            {
                for(i = 0; i != bytewidth; ++i) recon[i] = scanline[i];
                for(i = bytewidth; i < length; ++i) recon[i] = scanline[i] + (recon[i - bytewidth] >> 1);
            }
            break;
        case 4:
            if(precon)
            {
                for(i = 0; i != bytewidth; ++i)
                {
                    recon[i] = (scanline[i] + precon[i]); /*paethPredictor(0, precon[i], 0) is always precon[i]*/
                }
                for(i = bytewidth; i < length; ++i)
                {
                    recon[i] = (scanline[i] + paethPredictor(recon[i - bytewidth], precon[i], precon[i - bytewidth]));
                }
            }
            else
            {